The redox flow secondary battery stores and discharges electricity, and it belongs to a large-size stationary battery used for the equalization of electricity usage. In the redox flow secondary battery, an electrolytic solution comprising a positive electrode and a positive electrode active material (positive electrode cell) is separated from a negative electrolytic solution comprising a negative electrode and a negative electrode active material (negative electrode cell) by a separation membrane, and an oxidation-reduction reaction of the two active materials is utilized for the charge and discharge of electricity, and an electrolytic solution comprising each of the two active materials is supplied from a storage tank to an electrolytic cell, and then, electric current is obtained and used.
Examples of the active material contained in the electrolytic solution include iron-chromium-based materials, chromium-bromine-based materials, zinc-bromine-based materials, and vanadium-based materials that utilize a difference in electric charge.
In particular, since a vanadium battery has high electromotive force and it is advantageous in terms of a quick electrode reaction of vanadium ions, a small amount of hydrogen generated as a side effect, and high output power, the development of such a vanadium battery has vigorously proceeded.
In addition, the separation membrane is configured, such that an electrolytic solution comprising active materials for both electrodes is not mixed into the separation membrane.
For example, Patent Literature 1 discloses an example of a zinc-bromine-based redox flow battery having a separation membrane formed with a polyethylene porous membrane and a cation-exchange membrane, which is used to improve current efficiency.
Moreover, Patent Literature 2 discloses a separator for zinc-bromine-based battery having a substrate formed with macromolecules containing hydrophilic fine particles and an ion-exchange resin locally supported by a pore part of the substrate in a highly dispersed state, which are used to suppress the diffusion of ionized bromine molecules into a negative electrode side.